The present invention relates to a scanning electron microscope and a method for adjusting a focal point of an electron beam of said scanning electron microscope, for picking up an image of view upon an observation target, such as, industrial products, in particular, a semiconductor on way of production in preprocessing of semiconductors, through detection of electrons discharged from irradiation position, upon which a focused electron beam is irradiated, and in particular, it relates to a technology for reducing or shortening a tact time for picking-up a SEM image, especially, with using a SEM-type semiconductor wafer inspection apparatus, being required to pickup an image at high magnifying power, with a review SEM apparatus, for use of observation about defects, which are detected on the semiconductor wafer, in much more details thereof, and further with a measuring SEM apparatus, for measuring a pattern formed on the semiconductor wafer, but requiring no time for executing an automatic focusing thereof.
Accompanying miniaturization of the semiconductors, it comes be difficult, more and more, to control the manufacturing processes in preprocessing of such the semiconductors, and in particular, it is also difficult to detect and/or observe the defects, and to measure a size of pattern width, with using an optical microscope; therefore, the inspection, the review and measurement are commonly conducted, upon basis of an image which is pickup by means of the SEM apparatuses. Since the picked-up image through the SEM apparatus has a characteristic, of being deep in the focal depth comparing to that of the optical one, therefore, conventionally, the necessity is low for an automatic focusing thereof at high accuracy, in particular, with an inspection apparatus and/or a review apparatus of such SEM-type. This is because, the image-pickup target can be entered within the focal depth of the SEM, easily, through applying a method of detecting the height of the wafer surface by means of an optical height detector, which is described in the following Patent Document 1, for example, and thereby to fit the focus of the SEM apparatus upon basis of that surface height detected, but without applying the automatic focus based on the image, which may be picked up by means of that SEM apparatus.
As a prior art disclosing such the method as was mentioned above, there is already known the following Patent Document 2. As the technology disclosed in this Patent Document 2, there is described a method of picking up an image while moving a stage, as a manner for achieving a high-speed review apparatus; however, in this prior art, in particular, in relation to a manner of automatic focusing, it is presumed that a sensor is used for achieving real-time detection in the height direction of a wafer, with respect to the movement of the stage. However, in a case when picking up the observation target at further high magnifying power, since also the focal depth comes to be shallow, further; therefore, there occurs a necessity of an automatic focusing upon the basis of the image, which is picked up by means of the SEM. As a manner for achieving such the automatic focusing upon the basis of the image picked up through the SEM, in particular, in such the case, there is already known a method, including the following steps of, for example: picking up images at various settings while changing the setting of an electron optic system, so that the focus position of the SEM differs in Z direction, directing the focused electron beam irradiated from the SEM into the Z direction; applying a high-pass filter upon those images for passing through only components having high space-frequencies; determining that the focus is most fitted at the setting where an image can be picked up, at which setting this output response comes to be the largest; and achieving the automatic focusing upon the basis of the settings of the electron optic system which takes that image.
Patent Document 1: Japanese Patent Laying-Open No. Hei 11-183154 (1999); and
Patent Document 2: Japanese Patent Laying-Open No. 2002-183154 (2002).
Thus, with such the conventional arts as was mentioned above, there is a problem that an image at high magnification or high magnifying power cannot be picked up within a short tact time. As was explained in the above, the focal depth becomes shallow, when picking up an image of high magnification by means of the SEM apparatus. At the time of the year 2004, it comes down to be 90 nm, in particular, in width between lines formed on the semiconductor, therefore, for the purpose of measuring the thickness of this line or wire at high accuracy, there is a necessity of picking up an image at the magnifying power of about 150,000 times, for example. At that magnification, the resolution of about 2 nm is required to the electron optic system, and for achieving such the resolution, in general, the focal depth comes down to be less than 1 μm. Because of such shallow focal depth, in this manner, it is difficult to detect the surface height in the real-time manner. Also, if it is possible to achieve the real-time measurement on the surface height of the wafer with an error less than 1 μm, however, in general, it is impossible to achieve the focusing at high accuracy upon the basis of such the measurement value.
This is since, with the SEM apparatus, the image is picked up by detecting secondary electrons and/or electrons reflected, which are discharged from the observation target while irradiating the focused electron beam upon that observation target; however, if the balance is broken between the secondary electron beams irradiating thereupon and the secondary electrons or the electrons reflected from that observation target, in particular, in an amount thereof, then the observation target being charged with electricity; therefore, change will be caused easily in the focus position, with receiving ill influences of the electric field upon the surface of the observation target, which is caused due to such the charge of electricity. In particular, among the recent wafers, many of them are covered on the surface thereof, with an insulation film, such as, silicon oxide, for example, and if the number of electrons discharged comes up to be larger than that of those electrons being incident thereon, and then the wafer surface is charged with electricity to be positive, but because the material itself is an insulation film; therefore, it is difficult to supply depleted electrons form other areas of the wafer; therefore the charge of electricity has a tendency of becoming large in an amount thereof. For this reason, it is difficult to pick up an image in focus, upon the basis of a result of detection of the surface height, in particular, the wafer that is covered with the insulating film thereon.
On the other hand, with the method of picking up the images in plural numbers thereof, while shifting the focus in the Z direction, wherein the high-pass filter is applied for passing through only the components having high space-frequency upon the images, respectively, and it is determined that the focus is most fitted at the setting point where the image can be picked up, at which setting point this output response comes to be the largest, thereby, achieving the automatic focusing upon the basis of the setting points of the electron optic system which takes that image; however, there is a necessity of picking up the images in plural numbers thereof under the condition where the observation target and the electron optic system stand still at the positions thereof, so that no change occurs on the position within a filed of view (or view field) of the observation target, and for this reason, it brings about the problem of taking a long time for the contact. In a case where scanning is made upon the focused electron beam at the same frequency of an electron gun of a TV, for example, and if assuming a regular square view field, it needs a time of 33 ms or more or less.
In general, the images obtained from the SEM apparatus are noisy in quality thereof, in many cases, and therefore, for the purpose of improving the S/N thereof, it is common to apply a method thereupon, which is generally called by “flame addition”. With this method, through conducting the scanning of focused electron beam upon the view fields by plural numbers of times, so as to make up frames from the images obtained at this time, an averaged value is calculated out upon the plural numbers of frames of the same address, and this averaged value is reserved or stored as an image; thereby, reducing the noises down to 1/√{square root over (N)} when assuming that the number of addition of the frames is N. Now, assuming that the number of addition of the frames is four (4) for achieving the automatic focus, and then it takes 132 ms for picking up one (1) piece of image. Herein, assuming that ten (10) pieces of images are picked up while shifting the focus position into the Z direction, thereby to make up the automatic focusing, it takes about 1.3 sec for picking up the image; therefore, it is impossible to achieve the image pickup within a short tact time.